1. Field of the Invention
The invention relates to a microreactor, as is used in particular in an electrode arrangement of a fuel cell, and which can preferably also be constructed in the form of a so-called cartridge, in which fiber bundles, which are mounted in a cage between two head plates, are connected in parallel as individual electricity-producing cells.
2. Description of the Related Art
Both gases and liquids are used as fuel components for producing electricity in fuel cells, are supplied to at least two electrodes, an anode and a cathode, and are catalytically broken down there into ions and electrons. In this case, the transportation of ions must be physically separated from the transportation of electrons and, in general, the ions are transported within an electrolyte and the electrons are supplied and carried away by means of conductive electrodes, the cathode or cathodes and the anode or anodes, which are also referred to as collectors in this context.
The ion-conductive electrolytes can disassociate and in the process form H+ or OH— ions. Such electrolytes may be liquid materials such as alkaline aqueous alkali solutions, for example, KOH, or aqueous acids, for example, H2SO4, or solid ion-conducting materials, for example membranes. In this case, alkaline electrolytes conduct anions, while acidic electrolytes conduct cations.
Two types of fuel cells are favored at the moment, which both use oxygen as the fuel component, with the hydrogen cell using hydrogen as a further fuel component, and with the direct methanol cell, frequently referred to as a DMFC for short, using methanol as a further fuel component. In hydrogen cells, hydrogen is first of all produced by reformers from methanol, before it can be converted into electricity in the fuel cell. In direct methanol cells, methanol is supplied directly as the fuel component.
The reactions which occur in this case are as follows:
Hydrogen/oxygen cell:
    Anode reaction: H22H++2e− (oxidation)    Cathode reaction: O2+4H++4e−2H2O (reduction)MeOH/O2 cell:    Anode reaction: CH3OH+H2OCO2+6H++6e− (oxidation)    Cathode reaction: 1.5O2+6H++6e−3H2O (reduction)
U.S. Pat. No. 4,100,331 discloses a hydrogen fuel cell, in which hydrogen and oxygen are introduced, as fuel components, into separate, tubular membranes, which have a catalyst on the inside. Furthermore, an electrode is in each case arranged in a spiral shape within the membrane, with oxygen flowing around the cathode and hydrogen flowing around the anode, via which an electric current can be tapped off. The respective arrangements of the membrane, catalyst and electrode are aligned parallel to one another within an electrolyte in the fuel cell disclosed there.
U.S. Pat. No. 4,420,544 discloses a direct methanol cell in which, once again in an electrolyte, namely, H2SO4, a membrane is arranged with an internal catalyst coating. A spiral wire electrode is surrounded by the membrane, and oxygen flows against this wire electrode. Methanol is furthermore introduced into the electrolyte, and the disassociation process takes place between the spiral wire electrode and an electrode which is immersed in the electrolyte, is at a distance from the membrane and extends flat.
In a further exemplary embodiment, the electrode which is immersed in the electrolyte is in this case likewise spiral and is wound around the outer casing of the membrane.
In a comparable manner to a battery cell, such fuel cells can be connected in series in order to increase the voltage or in parallel in order to increase the power. Particularly when they are connected in parallel, the cartridges mentioned initially are expedient, in which membrane modules are used, such as those which are known from DE 44 38 327 and from DE 44 01 014.
Comparable circuits are known from WO 00/54358. There, a fuel component flows through a coaxial, tubular electrode arrangement. The microreactor on which this is based is in the form of a tubular assembly, in which a radially internal hollow mesh composed of bundles and/or wires of a material which conducts electrons forms an inner electrode, through which a fuel component flows. A catalyst layer is applied to the electrode, and a layer of a material which conducts ions is applied to this catalyst layer. This layer of a material which conducts ions in turn has a catalyst layer which is enclosed by a further hollow mesh composed of a material which conducts electrons, as the opposing electrode, with the different layers/materials being arranged such that they rest directly on one another.
A central metal wire with projections can be incorporated in the lumen of the internal hollow mesh, by means of which projections an electrical contact is produced between the wire and the internal hollow mesh, for a current output. Furthermore, the wire or wires is or are used to stabilize the tubular assembly.
In addition to the mechanical robustness, a problem which normally occurs in the fuel cell is that the power yield which it allows is only small since membrane surfaces, in particular, are normally limited in the same way as catalytic reaction areas, as well, in particular the so-called three-phase zone as a contact zone.